Gallons Per Minute Calculator (GPM)

The method

flow = volume ÷ time, or 2.448 × d² × velocity for a pipe

The 2.448 rule

For water in a round pipe, gallons per minute is about 2.448 × the inside diameter in inches squared × the velocity in feet per second. It is not an approximation but an exact unit conversion, and it is worth memorising: a 2-inch pipe at 5 ft/s carries about 49 gal/min.

Note the diameter is squared, so pipe size dominates. Doubling the diameter quadruples the flow at the same velocity — and because friction also falls, the real gain is larger still.

Keep velocity between about 5 and 8 ft/s. Slower and sediment settles out; faster and the pipe is noisy, loses pressure to friction, and erodes at the fittings.